i) Field of the Invention
The present invention relates to a superabsorbent polymer obtained by reacting a polysuccinimide (anhydropolyaspartic acid) with a diamine compound and subsequently hydrolyzing the reaction product or by reacting a polysuccinimide with a diamine compound in an aqueous solution and concurrently hydrolyzing the reaction product, and a process for producing the same.
ii) Description of the Prior Art
Polymer compounds referred to as superabsorbent polymers are known as polymers which absorb a large amount of water. These superabsorbent polymers are prepared from water-soluble polymers, but basically have crosslinking structures which render the polymers water-insoluble. Specific examples of the superabsorbent polymers include partial hydrolyzates of starch-acrylonitrile graft polymers, starch-acrylic acid graft polymers, crosslinked polymers formed by crosslinking acrylic acid with a copolymerizable crosslinking agent, hydrolyzates of methyl methacrylate-vinyl acetate copolymers, crosslinked polymers of polyvinyl alcohol base or polyethylene oxide base, and the like. Some of these have already been put to practical use [Masuda, F., "Superabsorbent polymers--characteristics and trends in development of application", Chem. Econ. Engineer. Rev., vol. 15, pp. 19-23 (1983)].
These superabsorbent polymers are widely used for fields including sanitary materials such as diapers and hygienic articles, agricultural materials such as water-holding materials for soil and sheets for seedlings, food-related materials such as food-freshness-keeping agents and dehydrating agents, public works materials or building materials such as dew inhibiting sheets on buildings and water sealing agents, and the like. Since these superabsorbent polymers are not hydrolyzed, they can exist in water or soil semipermanently.
This durability is a desirable property for some uses. However, it raises a problem in view of environmental preservation after disposal because the materials are not readily degraded to low molecules when used as throwaway items represented by sanitary materials such as diapers and hygienic articles.
On the other hand, there have been several reports or disclosures on the technique for preparing water-insoluble superabsorbent polymers by crosslinking polyamino acids which have excellent safety and degradability.
There have been a technique which comprises esterifying the carboxylic side chain of a monoaminodicarboxylic acid (acidic amino acid) such as polyaspartic acid or polyglutamic acid, and crosslinking the side chain with a diamine to form a water-insoluble superabsorbent polymer (Akamatsu et al., U.S. Pat. No. 3,948,863; Japanese Patent Publication No. 41309/1977). This technique, which employs as a starting material a polyamino acid with its carboxylic side chains esterified, differs from other prior art in hat the starting material is brought into contact with a multivalent amine to amidate a part of the ester groups in the side chains of the polyamino acid, and then a part or all of the unreacted ester groups are converted to carboxyl groups or salts thereof.
A water-insoluble poly(tyrosine-glutamic acid) was reported (Overell et al., Journal of Chemical Society, Part I, "Polymers of Some Basic and Acidic alpha-Amino-acids", pp. 232-236, 1955). However, no disclosure is made on the possibility of applying the formed polypeptide to a water-insoluble hydrogel and the technique of attaining a high water-absorbency.
A technique was reported, that comprises irradiating a poly-.gamma.-glutamic acid with .gamma. rays to crosslink the polymer so as to form a water-insoluble superabsorbent polymer [Kunioka et al., Kobunshi Ronbun-shu (Polymer Papers), vol. 50, No. 10, p. 755 (1993)]. From a scientific point of view, this technique is interesting in that the polyamino acid which is weak to heating can be reacted at a low temperature. From an industrial point of view, however, this technique is not realistic because the necessary Co-60 irradiation facility is a extremely large scale for shielding and needs a strict control of leaked .gamma. rays after irradiation. From an economical point of view, the superabsorbent polymer is disadvantageous because the polyglutamic acid used as the starting material is expensive.
There have been a technique which comprises crosslinking an acidic amino acid polymer with lysine-diketopiperazine to form a hydrophilic, biodegradable polymer (Iwatsuki et al., Japanese Patent Laid-Open No. 279416/1993). This technique however involves a problem such that preparation of the lysine-diketopiperazine as the crosslinking agent is difficult.
A proteinoid (protein-like compound) is produced from a mixture of .alpha.-amino acids by heat condensation at temperatures above 150.degree. C. [Fox and Harada, Science, vol. 128, p. 1214 (1958) and J. Am. Chem. Soc., vol. 82, pp. 3745-3751 (1959)]. In this synthesis, the presence of an excessive amount of a monoaminodicarboxylic acid (acidic amino acid) is necessary for an unknown reason. At a temperature of 210.degree. C. or higher, however, the amino acids are thermally decomposed so that the advantage of the excessive amount of a monoaminodicarboxylic acid is lost [Fox and Windsor, International Journal of Quantum Chemistry; Quantum Biology, vol. 11, pp. 103-108 (1984)].
There have been a technique which forms a water-soluble polypeptide by means of heat condensation (Fox et al., U.S. Pat. No. 4996292). However, no disclosure is made on the possibility of applying the formed polypeptide to a water-insoluble hydrogels and the technique of attaining a high water-absorbency.
A technique was reported, that crosslinks specific amino acid residues in an anionic polypeptide by means of heat condensation. In other words, there have been a technique which comprises heating glutamic acid and lysine or aspartic acid or polyaspartic acid and lysine to form a water-insoluble, crosslinked polypeptide (Japanese Patent National Publication No. 506244/1994; Donachy and Sikes, U.S. Pat. Nos. 5,247,068 and 5,284,936).
However, the reaction temperature disclosed of the heat condensation is 190.degree.-250.degree. C., that is severely high for the amino acids or polyamino acid. Accordingly, the formed polymer is unavoidably deteriorated in quality or colored due to partial decomposition and side reactions. The water-absorbency of the formed polymer is also insufficient. To improve the water-absorbency of the crosslinked polyamino acid formed, there have been a process in which the polyamino acid is hydrolyzed with an alkali at 80.degree.-95.degree. C. at a pH of 11-12 for 1-2 hours. This indicates that the cross-linked polyamino acid formed is resistant to the alkali treatment under such conditions, remaining as a water-insoluble hydrogel, and hence suggests that its degradability is low.